A repaired Concorde airliner roared away from Australia as it renewed its attempt at a supersonic speed record, leaving investigators baffled about why part of its tail tore off over the South Pacific last week. “There are no signs of metal fatigue,” said British Airways services manager Pat Boody. “Engineers have crawled all over it, replaced the tail assembly, and we still don’t know why it happened.” The jet lost a 3-by-4-foot chunk of its tail Wednesday on its dash from Christchurch, New Zealand, to Sydney, but it landed safely. The airliner is on a three-week odyssey that started April 1 in London. It is trying to establish supersonic speed records on each of 12 legs on its round-the-world journey.